Explore subscription groups button

...r/fragment/mainactivity/main/SubscriptionsFeedFragment.kt

...uto/platformplayer/views/subscriptions/SubscriptionBar.kt

...rmplayer/views/subscriptions/SubscriptionExploreButton.kt

app/src/main/res/drawable/background_button_explore.xml

...src/main/res/drawable/background_button_explore_inner.xml

app/src/main/res/drawable/sub_group_demo.png

app/src/main/res/layout/dialog_multi_button.xml

app/src/main/res/layout/view_subscription_bar.xml

...c/main/res/layout/view_subscription_group_bar_explore.xml

app/src/main/res/values/strings.xml